Please read Customize Webcit first.
WebCit uses the TinyMCE editor from MoxieCode Systems for composition of messages. It is an extremely powerful and versatile editor with many different configuration options -- in fact, the size of its code base exceeds that of WebCit itself.
WebCit's invocation of TinyMCE is templatized in a file called static/richedit.html . The default template invokes TinyMCE with a reasonable set of options for composing electronic mail and forum posts. If you wish to change the options, all you have to do is override richedit.html with your own version (such as static.local/richedit.html).
So, what kind of things can you customize? Pretty much everything: toolbar buttons, themes, optional modules, etc. Please refer to the TinyMCE documentation to find out all the things you can do. A list of tunable values can be found here.
If TinyMCE fails to initialize, your message editor will appear as a plain "textarea" field. To find out why it failed, open your browser's JavaScript console window and look for runtime errors.